1. Field of the Invention
This invention relates to an Al alloy brazing sheet which is excellent in corrosion resistance and is suitably used for assembling an Al alloy heat exchanger or the like for automobiles and various industrial purposes by bonding with brazing, particularly, vacuum brazing.
Incidentally, in this specification, the components of an Al alloy are all expressed by mass percent, which will be hereinafter abbreviated simply as %.
2. Description of the Prior Art
Brazing is effective in manufacturing an Al alloy structure of a complicated shape. In particular, vacuum brazing is extensively used, since this method requires no flux and is free from any environmental pollution problems. The vacuum brazing uses a brazing sheet in which one or both surfaces of a core material consisting of Al or Al alloy are clad with an Al alloy brazing filler metal as a cladding material. With this method, the brazing sheet is press-formed into members of various shapes, and these members are assembled in a predetermined structure such as a heat exchanger, for instance, which is then brazed by heating in a vacuum.
As for a brazing sheet for use in vacuum brazing, various Al alloys for a core material and various Al alloy brazing filler metals for a cladding material have been developed. At present, these materials are standardized by JIS-Z-3263 and JIS-H-4000.
In the case of an ordinary brazing sheet for use in vacuum brazing to manufacture a heat exchanger, JIS 3003 alloy (including, as a typical alloy, Al--Cu--Mn alloy with Cu content of 0.15% and Mn content of 1.1%), JIS 3005 alloy (including, as a typical alloy, Al--Mn--Mg alloy with Mn content of 1.1% and Mg content of 0.4%) and JIS 3105 alloy (including, as a typical alloy, Al--Mn--Mg alloy with Mn content of 0.6% and Mg content of 0.6%) are used for the core material, while JIS 4004 alloy brazing filler metal (including, as a typical material, Al--Si--Mg alloy with Si content of 10% and Mg content of 1.5%) and JIS 4104 alloy brazing filler metal (including, as a typical material, Al--Si--Mg--Bi alloy with Si content of 10%, Mg content of 1.2% and Bi content of 0.1%) are used for the cladding material. The brazing sheet has a thickness of about 0.3 to 1.5 mm, and one or both surfaces of the core material are clad with a brazing filler metal having a thickness 5 to 15% per side of the total thickness.
Such a brazing sheet is manufactured into an Al alloy heat exchanger having a hollow structure such as an evaporator of drawn-cup type, an oil cooler or a radiator. An evaporator so drawn-cup type 10 as shown in FIG. 4 for instance, is manufactured as follows. A brazing sheet, in which both surfaces of the above Al alloy core material are clad with the above Al alloy brazing filler metal, is manufactured into a refrigerant passage constituent member 1 as shown in FIGS. 2 and 3 by means of press-forming, and this member 1 is arranged in layers as shown in FIG. 4. Then, corrugated fins 2 are attached between the layered members 1, and side plates 6, 6', a refrigerant inlet pipe 7 and a refrigerant outlet pipe 8 are arranged and assembled into a core which is then brazed in a vacuum. Brazing in this case is carried out by heating up to about 873K in a vacuum of about 1.3.times.10.sup.3 to 1.3.times.10.sup.-2 Pa.
With a tendency to reduce the weight of automobiles, there have been demands that the thickness of materials for their heat exchangers should be reduced. On the other hand, automobiles are used in a variety of environments, and therefore, need materials which can withstand the various conditions encountered by automobiles. In particular, in case of a heat exchanger of drawn-cup type having no sacrificial layer, corrosion resistance of a core material of a brazing sheet is the most important characteristic. Thus, it is desirable to develop corrosion resistant brazing sheets.
The corrosion resistance of a brazing sheet is related to the distribution of alloying elements across the thickness of the material. However, alloying elements such as Cu and Zn, which are effective in corrosion resistance of a material, are distributed to be symmetrical with each other on the sheet surface side and its reverse side about the center of the sheet thickness when heating for brazing is carried out. Therefore, when the surface of a brazing sheet causes pitting corrosion, a material, which makes progress of pitting corrosion on the surface side slower than that at the center of the thickness, starts acceleration of pitting corrosion subsequent to that at the center of the thickness. Thus, it is not too much to say that a portion corresponding to half of the thickness makes a contribution to the improvement of corrosion resistance. Improvement of penetration life by adding alloying elements to a core material has its limit.
Since Cu added makes the potential of aluminum noble, Cu is added to the core material of a brazing sheet for the purpose of improving the corrosion resistance. However, Cu added to the core material is diffused into a brazing filler metal which has melted in heating for brazing. Cu further forms an eutectic and is concentrated when the brazing filler metal is solidified after heating for brazing. Therefore, the more Cu the core material of a brazing sheet contains, the more the corrosion of a layer resulting from resolidifying the brazing filler metal is accelerated. Accordingly, the quantity of alloying elements such as Cu added to the core material of the brazing sheet is limited for the improvement of corrosion resistance.
Conventionally, a four or five-layer brazing sheet having an intermediate layer between a core material and a brazing filler metal has been used for the purpose of preventing alloying elements of the brazing filler metal from being diffused into the core material, or preventing alloying elements of the core material from being diffused into the brazing filler metal in heating for brazing. For instance, brazing sheets disclosed in Japanese Patent Toku-Kai-Hei 2-30394, 1-208432 and 5-65582 or the like correspond to such a four or five-layer brazing sheet.
However, these brazing sheets as disclosed above do not sufficiently improve the exterior corrosion resistance of a heat exchanger (i.e., an evaporator of drawn-cup type or the like).